A monthly electronic newsletter summarizing Minnesota climate conditions and the
resulting impact on water resources.

Distributed on the Wednesday following the first Monday of each month.

State Climatology Office - DNR Waters

compiled 9/6/00

WHAT HAS HAPPENED:

- rainfall was generally near to below normal for most of Minnesota during the
month of August. Some northwestern and north central Minnesota counties were exceptions to
this general pattern and reported August precipitation totals an inch or more above the
norm. Additionally, isolated heavy August thunderstorms brought larger amounts of rain to
some small areas of southern Minnesota. Rochester set an all-time record for total
rainfall for the months of May through August. The May through August total of 30.70
inches breaks the old record for the same period set in 1990 of 27.38 inches.
(see: http://climate.umn.edu/cawap/monsum/monsum.asp
, http://climate.umn.edu/doc/weekmap.asp)
- a significant rainfall event occurred after the end of August, but before the posting of
this note. Therefore, it will be mentioned here in the August summary. During the evening
of September 2, very heavy thunderstorms formed just to the west of the Twin Cities
metropolitan area and dropped three to four inches of rain in less than three hours across
portions of Ramsey County. A six inch rainfall total was reported in northern Ramsey
County. During a one hour period, 2.60 inches of rain fell on the University of Minnesota
- St. Paul campus. A one hour total of 2.60 inches is approximately a "50 year
event" for the Twin Cities (see "FROM THE AUTHOR" below).
- August temperatures were near the historical average across Minnesota. The state
experienced a number of sultry days, with dew point temperatures climbing into the 70's on
six occasions.
(see: http://climate.umn.edu/cawap/monsum/monsum.asp
, http://climate.umn.edu/doc/whatsnew.htm)
- while not directly tied to hydrology, a notable atmospheric phenomenon occurred on
August 7. A destructive tornado struck in and near the city of Lakefield (Jackson county),
causing significant damage. In another tragic weather-related item, a man was killed in
Cass county on August 14 when thunderstorm winds caused a tree branch to fall upon his
tent.

WHERE WE STAND NOW:

- in an area of central and east central Minnesota, extending from roughly St. Cloud to
the northern suburbs of the Twin Cities, growing season (April 1 - present) rainfall
totals are short of normal by more than 30%. Unlike much of the rest of the state, this
area has never fully rebounded from the precipitation deficits accrued during the fall,
winter, and early spring. For example, total precipitation for Santiago (northern
Sherburne county) is approximately nine inches below normal for the last 12 months. (See
note below from Professor Bob Weisman of St. Cloud State University concerning central
Minnesota dryness.) Pockets of dryness also exist along the north shore of Lake Superior,
especially Cook county. In contrast, late summer rainfalls have pushed growing season
precipitation totals in much of northwestern Minnesota to 25% or more above the historical
average. This is a continuation of the wet regime this area has seen over much of the last
10 years. As noted earlier about Rochester, growing season precipitation in south central
and southeastern Minnesota remains well above historical averages, with some locations
falling at or above the 98th percentile for April 1 through September 5 rainfall.
(see: http://climate.umn.edu/doc/weekmap.asp)
- as of their August 31 release, the National Drought Mitigation Center - "U.S.
Drought Monitor" does not classify any Minnesota region in a drought category. The
NDMC index is a blend of science and subjectivity where intensity categories are based on
six key indicators and numerous supplementary indicators. The NDMC depiction attempts to
describe drought conditions on a fairly large regional scale, and has difficulty capturing
geographically isolated precipitation deficits such as those currently found in portions
of central and east central Minnesota (see discussion in Palmer Drought Severity Index
section below).
(see http://enso.unl.edu/monitor/monitor.html)
- the September 2nd Palmer Drought Severity Index (PDSI) depicts most of Minnesota in the
"Near Normal" category. Counties in south central and southeastern Minnesota are
categorized as experiencing an "Unusual Moist Spell". The Palmer Drought
Severity Index is used for assessing long-term meteorological conditions. One of the
inherent weaknesses of the PDSI is the size of the geographic regions ("climate
districts") covered by the analysis. The persistent dryness found in portions of
central and east central Minnesota is "washed out" by relative wetness in other
counties within the same climate district. Thus in this case, the PDSI fails to illustrate
areas of Minnesota experiencing significant precipitation deficits. For this reason, it is
always useful to cross-check the PDSI depictions with the State Climatology Office
seasonal precipitation maps.
(see http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/regional_monitoring/palmer.gif
,http://climate.umn.edu/doc/weekmap.asp)
- the Minnesota Agricultural Statistics Service reports that northwestern Minnesota has
surplus topsoil moisture, while the southern half of the state has areas of very short
topsoil moisture. Wet soils in northwestern Minnesota have hindered fall harvesting
operations whereas crops in moisture stressed fields of southern Minnesota are not
ripening normally. Topsoil moisture conditions across the state were rated 14% very short,
30% short, 50% adequate, and 6% surplus as of Friday, September 1.
(see: http://www.nass.usda.gov/mn/cwmn.htm
, http://climate.umn.edu/img/soil_moisture/wassm12.gif
, http://swroc.coafes.umn.edu/Weather/Charts/Soil/00_soil_water.html)
- current discharge values for most Minnesota streams rank in the "normal flow"
range for this time of year. However, for some portions of central and east central
Minnesota, and in one basin in southwestern Minnesota, stream flows are in the "low
flow" and "protected flow" categories. Conversely, stream discharge ranks
in the "high flow" category in some northwestern and southeastern watersheds.
(see http://water.usgs.gov/cgi-bin/daily_flow?mn
, http://www.dnr.state.mn.us/waters/programs/surwat_section/stream_hydro/productsf.html)
- the wildfire danger potential is rated as "moderate" for a ring of counties
just north of the metropolitan area, and in Cook county. The "blow down" zones
of the BWCA are rated in the "high" fire danger category.
(see: http://www.dnr.state.mn.us/forestry/fire/)

FUTURE PROSPECTS:

- the 30-day outlook from the Climate Prediction Center calls for near normal September
precipitation statewide. Normal September precipitation ranges from near two inches in far
western Minnesota, to over three inches in the east. The September temperature outlook
tilts towards above normal conditions statewide. Normal September high temperatures fall
off rapidly from the low 70's early in the month to the low 60's by month's end. Normal
September lows drop from the low to mid 50's early in the month to the upper 30's and low
40's by late September. The average date of the first occurrence of 32 degrees or lower
("frost date") falls within the second week of September in some low-lying areas
of far north central and northeastern Minnesota, the third week of the month for the
remainder of the northern third of the state, and the last week of September for the
northern two thirds of Minnesota. The average frost date for the southern one third of
Minnesota occurs during the first week of October.
(see: http://www.cpc.ncep.noaa.gov/products/predictions/multi_season/13_seasonal_outlooks/color/seasonal_forecast.html
,http://climate.umn.edu/text/historical/frost.txt)
- the 90-day outlook for September through November indicates near normal precipitation
statewide. The September through November temperature also calls for near normal
conditions throughout Minnesota.
(see: http://www.cpc.ncep.noaa.gov/products/predictions/multi_season/13_seasonal_outlooks/color/seasonal_forecast.html)

FROM THE AUTHOR:
(repeated from last month)

- one of the more confusing phrases used in meteorology and hydrology is "100-year
storm". The phrase implies that an intense rain storm dubbed as a 100-year event
brings rainfall totals heretofore unseen for 100 years, and not to be experienced again
for another century. This is a logical, but incorrect conclusion to draw from the phrase.
A "100-year storm" drops rainfall totals having a one percent probability of
occurring at that location that year. Encountering a 100-year storm on one day does
nothing to change the odds of seeing the same amount of precipitation the very next day. Intense rainfalls are typically geographically isolated. Therefore, increased
population density and improved precipitation monitoring networks have increased the
likelihood of capturing (measuring) heavy rain events. Also, improved communication allows
faster and more complete transfer of weather information. When the neighboring county is
walloped by a "100-year storm", we hear about it quickly. Invariably we will
vicariously "experience" the event and wonder why "100-year storms"
seem to be occurring every month! July's flash flood event in northern Dakota county
occurred in an area struck by a storm of similar magnitude in July of 1987. In both
downpours, the precipitation totals far exceeded the "100 year storm" design
threshold of six inches for the area. The communities affected by these storms tragically
beat the odds twice in 13 years.

The dryness, which has plagued a small piece of central Minnesota from eastern Stearns,
southeastern Morrison, and Benton Counties to the northern Twin Cities Metro, intensified
in August, according to the monthly statistics from the Saint Cloud Airport. August's
rainfall was only 1.21 inches, 2.75 inches below normal. The low rainfall made August 2000
the driest August since August 1981 (no measurable rain recorded) and the 10th driest
August of the 109 years on record. Almost all of the monthly rainfall occurred in a cool
storm on the 16th when 1.00 inch fell. Occasional high heat, especially during the first
portion of the month, put additional strain on water resources and forced restrictions on
outdoor water use in Sauk Rapids, Sartell, and other communities. The low rainfall
contributed to the low summer (1 Jun-31 Aug) total of 7.56 inches, the 16th lowest total
in St. Cloud records. While both 1992 (7.49 inches) and 1996 (6.74 inches) had lower
summer rainfalls, the 2000 rainfall was lower than in the drought years of 1988 (8.14
inches) and 1999 (8.29 inches). This brought the growing season rainfall (1 Apr-31 Aug) to
only 11.53 inches, more than 5 1/2 inches below normal.

- from Mike Mueller, DNR Waters Area Hydrologist - Cambridge

Total precip for August at Cambridge was 1.24". From July 12 to the current, we've
received about 2" total.